A frame structure under a TDD mode in the Long Term Evolution (LTE) system (also called frame structure type 2) is shown in FIG. 1. In this frame structure, a radio frame of a length of 10 ms (307200 Ts, 1 ms=30720 Ts) is divided into two half-frames, the length of each half-frame is 5 ms (153600 Ts), and each half-frame contains 5 subframes of a length of 1 ms each. The role of each subframe is shown in Table 1, wherein D denotes a downlink subframe used to transmit downlink signals; and U denotes an uplink subframe used to transmit uplink signals. Further, an uplink/downlink subframe is divided into two time slots of a length of 0.5 ms each. S denotes a special subframe containing three special time slots which are a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Time Slot (UpPTS). In a practical system, the mobile phones are informed of the indexes of uplink/downlink configuration through broadcast messages.
TABLE 1Uplink/Downlink configurationConfig-Switch-pointSubframe numberurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The structure of the Physical Random Access Channel (PRACH, also called Random Access Opportunity) in the LTE system is shown in FIG. 2. A preamble consists of a CP and a Sequence. Different preamble formats mean different lengths of CPs and/or Sequences. Currently, the preamble formats supported by the TDD mode in the LTE system are shown in Table 2.
TABLE 2Preamble formatsPreamble formatTCPTSEQ0 3168 · Ts24576 · Ts121024 · Ts24576 · Ts2 6240 · Ts2 · 24576 · Ts321024 · Ts2 · 24576 · Ts4 (frame structure 448 · Ts 4096 · Tstype 2 only)
Among the foregoing preamble formats, preamble formats 0-3 are transmitted in the normal uplink subframes, while preamble format 4 is transmitted in the UpPTS.
Preamble format 0 is transmitted in one normal uplink subframe;
Preamble format 1 or 2 is transmitted in two normal uplink subframes;
Preamble format 3 is transmitted in three normal uplink subframes;
Preamble format 4 is transmitted in the UpPTS (the starting position for transmitting is the position which is 5158 Ts before the end of UpPTS).
A Resource Block (RB) is used as a unit in the resource assignment in the LTE system. An RB occupies 12 Resource Elements (REs) in the frequency domain and occupies one time slot in the time domain, i.e. occupies 7 SC-OFDM symbols of normal cyclic prefix (normal CP) or 6 SC-OFDM symbols of extended cyclic prefix (extended CP). If the total number of RBs to which the uplink system bandwidth corresponds in the frequency domain is defined as NRBUL, then the indexes of RBs will be 0, 1, . . . , NRBUL−1 and the indexes of the sub-carriers (or called REs, namely Resource Elements) will be 0, 1, . . . , NRBUL·NSCRB−1. NSCRB is the number of sub-carriers to which a RB corresponds in the frequency domain.
In the frequency domain, a PRACH occupies the bandwidth to which 6 RBs correspond, i.e. 72 REs. The bandwidth of each RE is 15 kHz. The PRACHs with the same time domain positions are differentiated by the frequency domain.
An UpPTS in the TDD system may be used to send an SRS of uplink channel and a PRACH of Preamble format 4.
The method of alternate one-side mapping is adopted in the frequency domain mapping of the PRACH sent in the UpPTS, i.e. mapping from the low frequency band to the high frequency band in certain UpPTS and mapping from the high frequency band to the low frequency band in a neighboring UpPTS. The mapping formula may be expressed in the following form.
      n    PRB    RA    =      {                                                                      n                                  PRB                  ⁢                                                                          ⁢                  offset                                RA                            +                              6                ⁢                                                                  ⁢                                  f                  RA                                                      ,                                                              if              ⁢                                                          ⁢                              (                                                                                                                              (                                                                                    n                              f                                                        ⁢                            mod                            ⁢                                                                                                                  ⁢                            2                                                    )                                                ×                                                                                                                                                                          (                                                      2                            -                                                          N                              SP                                                                                )                                                +                                                  t                          RA                          1                                                                                                                    )                            ⁢              mod              ⁢                                                          ⁢              2                        =            0                                                                                          N                RB                UL                            -              6              -                              n                                  PRB                  ⁢                                                                          ⁢                  offset                                RA                            -                              6                ⁢                                                                  ⁢                                  f                  RA                                                      ,                                    otherwise                    
wherein nPRBRA denotes the index of the first RB of the PRACH; nPRB offsetRA is the frequency domain starting position of the PRACH; NRBUL is the total number of RBs to which the uplink system bandwidth configuration corresponds; fRA is the frequency domain index of the PRACH with the same time domain position; nf is the radio frame number, and NSP is the number of the switch-points from downlink to uplink in a radio frame of a length of 10 ms. tRA1=0,1 denote assignment in the first half-frame and the second half-frame of a radio frame respectively. There may be a plurality of PRACHs in an UpPTS. These PRACHs are continuous in the frequency domain.
SRS-Bandwidth configuration is based on a tree structure. Each SRS bandwidth configuration corresponds to a tree structure, the SRS-Bandwidth of the highest level corresponds to the maximum bandwidth of this SRS bandwidth configuration. Tables 3-6 show the SRS bandwidth configurations in different uplink bandwidth ranges. mSRS,b denotes the number of RBs which correspond to the SRS-Bandwidth of the level whose index is b in the tree structure, Nb denotes the number of branch nodes of a node on the level whose index is (b−1), which are located on the level whose index is b in the tree structure, b=0 corresponds to the first level, namely the highest level of the tree structure, and mSRS,0 is the maximum SRS-Bandwidth under this configuration. N/A denotes that there are not any corresponding branch nodes on the level.
Taking the SRS bandwidth configuration 1 in Table 3 for example, b=0 is the first level, the SRS-Bandwidth to which this level corresponds is the bandwidth to which 32 RBs correspond, and is the maximum SRS-Bandwidth of this SRS bandwidth configuration; b=1 is the second level, the SRS-Bandwidth of this level is the bandwidth to which 16 RBs correspond, and the SRS-Bandwidth of the first level is divided into two SRS-Bandwidths of the second level; b=2 is the third level, the SRS-Bandwidth of this level is the bandwidth to which 8 RBs correspond, and the SRS-Bandwidth of the second level is divided into two SRS-Bandwidths of the third level; b=3 is the fourth level, the SRS-Bandwidth of this level is the bandwidth to which 4 RBs correspond, and the SRS-Bandwidth of the third level is divided into two SRS-Bandwidths of the fourth level.
Further, the sub-carriers of the SRS in the same SRS frequency band are located in every other sub-carrier. As shown in FIG. 4, this comb structure allows more users to send SRSs in the same SRS-Bandwidth.
TABLE 3(6 ≦ NRBUL ≦ 40)SRS-SRS-SRS-SRS-BandwidthBandwidthBandwidthBandwidthSRSb = 0b = 1b = 2b = 3bandwidth configurationmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb0361123N/A143132116282422241N/A1N/A1463201N/A1N/A1454161N/A1N/A1445121N/A1N/A143681N/A1N/A142741N/AN/AN/AN/AN/AN/A
TABLE 4(40 < NRBUL ≦ 60)SRS-SRS-SRS-SRS-SRSBandwidthBandwidthBandwidthBandwidthbandwidthb = 0b = 1b = 2b = 3configurationmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb048124212 243148116382422401202N/A1453361123N/A143432116282425241N/A1N/A1466201N/A1N/A1457161N/A1N/A144
TABLE 5(60 < NRBUL ≦ 80)SRS-SRS-SRS-SRS-SRSBandwidthBandwidthBandwidthBandwidthbandwidthb = 0b = 1b = 2b = 3configurationmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb0721243122431641322162442601203N/A1453481242122434481163 82425401202N/A1456361123N/A1437321162 8242
TABLE 6(80 < NRBUL ≦ 110)SRS-SRS-SRS-SRS-SRSBandwidthBandwidthBandwidthBandwidthbandwidthb = 0b = 1b = 2b = 3configurationmSRS,bNbmSRS,bNbmSRS,bNbmSRS,bNb0961482242461961323162442801402202453721243122434641322162445601203N/A1456481242122437481163 8242
In the UpPTS, when the maximum SRS-Bandwidth is used to send an SRS, the maximum SRS-Bandwidth mSRS,0max may also be calculated with the following formula (namely the SRS-Bandwidths to which b=0 corresponds in Tables 3-6 may not be used):
      m          SRS      ,      0        max    =            max                        α          2                ,                  α          3                ,                  α          5                      ⁢          [                        m                      SRS            ,            0                          =                                            2                              1                +                                  α                  2                                                      ·                          3                              α                3                                      ·                          5                              α                5                                              ❘                                    m                              SRS                ,                0                                      ≤                          (                                                N                  RB                  UL                                -                                  6                  ·                                      N                    RA                                                              )                                          ]      
wherein NRBUL is the number of RBs to which the uplink system bandwidth corresponds; NRA is the number of PRACHs in the UpPTS; α2, α3 and α5 are non-negative integers. This formula means that the maximum SRS-Bandwidth meeting mSRS,0≦(NRBUL−6·NRA) can be obtained through selecting the values of α2, α3 and α5.
When an SRS is sent, the SRS-Bandwidth of each level of the tree structure is within the frequency band range of the maximum SRS-Bandwidth, and the relative position where the SR-bandwidth of each level is located in the maximum SRS-Bandwidth is changeable. Therefore, in order to prevent the SRS from interfering with the PRACH in the UpPTS and implement the channel sounding for more bandwidth, the frequency domain position of the maximum SRS-Bandwidth in the UpPTS should be configured reasonably.